When running at idle, an internal combustion engine is only lightly loaded and therefore ingests fuel at a rate that is small in comparison to rates that are required at higher speeds and loads. When fuel is introduced into the engine cylinders by means of an individual electronically controlled fuel injector for each cylinder, each injector is required to operate over a rather extensive range of opening and closing times. In order to operate the engine at high speeds and loads, it is vital that each injector have the ability to flow fuel at a certain flow rate; yet at idle, a much lower flow rate is used. Stated another way, such an injector is required to have a relatively large dynamic range. Where a particular injector is designed for a specific maximum flow rate, it may be difficult for such an injector to accurately inject fuel at the low end of the required range. This difficulty is amplified in a two-stroke engine.
A further consideration related to a two-stroke engine involves the matter of scavenging. The inherent nature of the design of a two-stroke engine leaves a significant amount of residual combustion products in a combustion chamber as the chamber is being prepared for the immediately succeeding combustion event. The presence of such residual products influences the nature of the combustion process, and when a two-stroke engine is used as the powerplant of an automotive vehicle, factors such as fuel economy and exhaust emissions are affected. A known means of improving scavenge efficiency and increasing the quantity of fuel injected per cycle is to retard the spark timing.
The present invention relates to means and methodology for improving the operation of a multi-cylinder fuel-injected two-stroke internal combustion engine at idle and off-idle. The invention involves the deliberate skipping of injection cycles in particular patterns which serve to create modest, but nonetheless meaningful, improvements in operating efficiency and exhaust emissions without causing any noticeable degradation in the quality of the engine's operation at idle. Briefly, the pattern is such that over a certain number of engine crankshaft revolutions the interruptions of fuel injection into each individual cylinder are caused to occur at non-consecutive two-stroke cycles and the interruptions in the sequence of injection from cylinder to cylinder are caused to occur non-consecutively. Each interrupted injection results in the introduction of air alone into the associated cylinder on the immediately succeeding cycle whereby the residual combustion products are diluted by the charge of air. The scavenging that occurs after the interrupted fuel injection cycle therefore results in a cylinder that is much better purged of combustion products before the next combustion event that takes place in that cylinder. Accordingly, that combustion event will make more efficient use of the injected charge of fuel.
Since the idle load that is imposed on the engine requires a certain power output from the engine, the skipping of certain injection cycles at idle means that on the average each combustion event in each cylinder must produce a higher power output in comparison to the situation where injection cycles are not skipped. This higher power output is accomplished by causing each injector to flow a correspondingly higher amount of fuel when the injection skipping pattern is in effect at idle. Two benefits result from the invention. One, it means that the lower limit of the fuel injectors' dynamic ranges does not have to be as low as in the case of non-skipping, and two, it means that the spark timing can be advanced over the value used for non-skip operation. Reducing the dynamic range requirement of a fuel injector is an advantage for obvious reasons, and the advancement of spark timing of course promotes better combustion efficiency and fuel economy.
The features of the invention that have been mentioned above, along with further ones, will be seen in the ensuing detailed description of a presently preferred embodiment of the invention. The description includes the best mode contemplated at the present time for the practice of the invention. As an aid to explaining the inventive principles, a drawing accompanies the disclosure.